Now, I want to introduce the technique called edge bundling. You may have noticed that in the previous video, the last image, we showed this nice circular graph layout, where the edges or the lines were not straight, they had these very nice curves. The technique used there is exactly this one, that is called edge bundling. Edge bundling is very interesting technique. The idea is that when we have too many lines and these lines are crossing and creating a lot of clutter, we can reduce the clutter and hopefully facilitate the perception of the direction of this lines by bundling them together. All of the lines that goes in the similar directions get bundled together. These are very nice technique that has been created by Danny Holten in 2006. What you see here is one example of using edge bundling, coming from the paper then he published back then. As you can see in the image on the left, you have the version with straight lines and the image on the right you have what happens when you use edge bundling. As you can see is much less cluttered and much easier to follow the main paths and what lines goes exactly in similar directions or follow similar paths. So, how does edge bundling work? I want to give you some little understanding of what's the basic idea behind it. The basic idea behind it is that rather than drawing a straight line between the two points, the line follows a path that is designed by some structure that connects the nodes together. Here in this example, you see very small version of how edge bundling works. The image that you see on the right shows you the structure that has been designed, hierarchical structure that connects all the points. At the beginning in the image on the left, you can see that there is a straight line between the two points that wants to be connected. Now, rather than drawing these straight lines, the idea is to use a method that interpolates between the points of the hierarchy of the structure, so that all the lines that go through similar paths get bundled together. So, what you see on the top right hand side, is the path that the line has to follow to the what is called the control points. Then, the lines are drawn using what is called the spline, which is computer graphics methods to draw a very nice curved line. So, that's the main idea and the image that you see at the bottom is what happens when you use this technique. The idea is that the structure that is depicted there is made of a number of control points and these control points are the control points that actually create the spline basically. One problem that may happen with this technique, is that if lines that follow similar path pass exactly through the same path, may be occluded. So, an additional interesting technique that has been added in the method, is the idea of adding different factors to group together or disentangle lines that pass through similar positions and this is what you see exactly here. So, here is another example of using edge bundling on a whole visualization, and what you see on the left is the hierarchical structure that is used to create the control points and what you see on the right is how the lines get bundled together through the control points that are in the structure that you see on the left. If you're curious about how actually one can use this technique for the original visualization that I've shown you, the radial chart, so here is a more specific example. On the left, you see similar radial chart, where the lines are drawn just straight between the points. As you can see, there is a structure, you always need structure in order to create edge bundling and there is just one single point in the middle of the circle and the elements that are grouped together are connected with lines and the points pass through. The control points are those that actually create the smooth lines, that bundled the lines together and this is what you see on the right hand side. So, this is focusing on only two edges, so as you can see they are straight and once they are bundled, they pass through these control points.
